Conventional range detectors are available in frequency-modulated continuous-wave implementations but these are expensive and complex. A simpler approach of using the relative phase shift between two signals of different frequency or wavelength is known but is not practical because beyond a phase shift of 180.degree. there is ambiguity. That is, a particular phase difference could indicate either a close target moving toward the antenna or a distant target moving away from the antenna, or the converse. Attempts to resolve this ambiguity by detecting the difference in the strength of the signal reflected from the target at a distance represented by a phase shift of under 180.degree. from one over 180.degree. is impractical because the signals are small and difficult to distinguish, making the device unreliable and unattractive for most applications. There are many applications where the target is moving and high reliability is required yet conventional sensors are precluded because of their cost and complexity. Other technologies such as ultrasonics are too sensitive to environmental conditions.